Add support for partial custom editor themes

doc/classes/Theme.xml

@@ -275,6 +275,14 @@
 				Returns [code]false[/code] if the theme does not have [code]node_type[/code].
 			</description>
 		</method>
+		<method name="merge_with">
+			<return type="void" />
+			<argument index="0" name="other" type="Theme" />
+			<description>
+				Adds missing and overrides existing definitions with values from the [code]other[/code] [Theme].
+				[b]Note:[/b] This modifies the current theme. If you want to merge two themes together without modifying either one, create a new empty theme and merge the other two into it one after another.
+			</description>
+		</method>
 		<method name="rename_color">
 			<return type="void" />
 			<argument index="0" name="old_name" type="String" />

editor/editor_themes.cpp

@@ -571,6 +571,11 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 	theme->set_stylebox("panel", "PanelContainer", style_menu);
 	theme->set_stylebox("MenuPanel", "EditorStyles", style_menu);
 
+	// CanvasItem Editor
+	Ref<StyleBoxFlat> style_canvas_editor_info = make_flat_stylebox(Color(0.0, 0.0, 0.0, 0.2));
+	style_canvas_editor_info->set_expand_margin_size_all(4 * EDSCALE);
+	theme->set_stylebox("CanvasItemInfoOverlay", "EditorStyles", style_canvas_editor_info);
+
 	// Script Editor
 	theme->set_stylebox("ScriptEditorPanel", "EditorStyles", make_empty_stylebox(default_margin_size, 0, default_margin_size, default_margin_size));
 	theme->set_stylebox("ScriptEditor", "EditorStyles", make_empty_stylebox(0, 0, 0, 0));
@@ -1376,15 +1381,14 @@ Ref<Theme> create_editor_theme(const Ref<Theme> p_theme) {
 }
 
 Ref<Theme> create_custom_theme(const Ref<Theme> p_theme) {
-	Ref<Theme> theme;
+	Ref<Theme> theme = create_editor_theme(p_theme);
 
-	const String custom_theme = EditorSettings::get_singleton()->get("interface/theme/custom_theme");
-	if (custom_theme != "") {
-		theme = ResourceLoader::load(custom_theme);
-	}
-
-	if (!theme.is_valid()) {
-		theme = create_editor_theme(p_theme);
+	const String custom_theme_path = EditorSettings::get_singleton()->get("interface/theme/custom_theme");
+	if (custom_theme_path != "") {
+		Ref<Theme> custom_theme = ResourceLoader::load(custom_theme_path);
+		if (custom_theme.is_valid()) {
+			theme->merge_with(custom_theme);
+		}
 	}
 
 	return theme;

editor/plugins/canvas_item_editor_plugin.cpp

@@ -4198,6 +4198,10 @@ void CanvasItemEditor::_notification(int p_what) {
 		font->set_outline_color(Color(0, 0, 0));
 		zoom_reset->add_font_override("font", font);
 		zoom_reset->add_color_override("font_color", Color(1, 1, 1));
+
+		info_overlay->get_theme()->set_stylebox("normal", "Label", get_stylebox("CanvasItemInfoOverlay", "EditorStyles"));
+		warning_child_of_container->add_color_override("font_color", get_color("warning_color", "Editor"));
+		warning_child_of_container->add_font_override("font", get_font("main", "EditorFonts"));
 	}
 
 	if (p_what == NOTIFICATION_VISIBILITY_CHANGED) {
@@ -5778,20 +5782,13 @@ CanvasItemEditor::CanvasItemEditor(EditorNode *p_editor) {
 	info_overlay->add_constant_override("separation", 10);
 	viewport_scrollable->add_child(info_overlay);
 
+	// Make sure all labels inside of the container are styled the same.
 	Theme *info_overlay_theme = memnew(Theme);
-	info_overlay_theme->copy_default_theme();
 	info_overlay->set_theme(info_overlay_theme);
 
-	StyleBoxFlat *info_overlay_label_stylebox = memnew(StyleBoxFlat);
-	info_overlay_label_stylebox->set_bg_color(Color(0.0, 0.0, 0.0, 0.2));
-	info_overlay_label_stylebox->set_expand_margin_size_all(4);
-	info_overlay_theme->set_stylebox("normal", "Label", info_overlay_label_stylebox);
-
 	warning_child_of_container = memnew(Label);
 	warning_child_of_container->hide();
 	warning_child_of_container->set_text(TTR("Warning: Children of a container get their position and size determined only by their parent."));
-	warning_child_of_container->add_color_override("font_color", EditorNode::get_singleton()->get_gui_base()->get_color("warning_color", "Editor"));
-	warning_child_of_container->add_font_override("font", EditorNode::get_singleton()->get_gui_base()->get_font("main", "EditorFonts"));
 	add_control_to_info_overlay(warning_child_of_container);
 
 	h_scroll = memnew(HScrollBar);

scene/resources/theme.cpp

@@ -1182,6 +1182,82 @@ void Theme::copy_theme(const Ref<Theme> &p_other) {
 	_unfreeze_and_propagate_changes();
 }
 
+void Theme::merge_with(const Ref<Theme> &p_other) {
+	if (p_other.is_null()) {
+		return;
+	}
+
+	_freeze_change_propagation();
+
+	// Colors.
+	{
+		const StringName *K = nullptr;
+		while ((K = p_other->color_map.next(K))) {
+			const StringName *L = nullptr;
+			while ((L = p_other->color_map[*K].next(L))) {
+				set_color(*L, *K, p_other->color_map[*K][*L]);
+			}
+		}
+	}
+
+	// Constants.
+	{
+		const StringName *K = nullptr;
+		while ((K = p_other->constant_map.next(K))) {
+			const StringName *L = nullptr;
+			while ((L = p_other->constant_map[*K].next(L))) {
+				set_constant(*L, *K, p_other->constant_map[*K][*L]);
+			}
+		}
+	}
+
+	// Fonts.
+	{
+		const StringName *K = nullptr;
+		while ((K = p_other->font_map.next(K))) {
+			const StringName *L = nullptr;
+			while ((L = p_other->font_map[*K].next(L))) {
+				set_font(*L, *K, p_other->font_map[*K][*L]);
+			}
+		}
+	}
+
+	// Icons.
+	{
+		const StringName *K = nullptr;
+		while ((K = p_other->icon_map.next(K))) {
+			const StringName *L = nullptr;
+			while ((L = p_other->icon_map[*K].next(L))) {
+				set_icon(*L, *K, p_other->icon_map[*K][*L]);
+			}
+		}
+	}
+
+	// Shaders.
+	{
+		const StringName *K = nullptr;
+		while ((K = p_other->shader_map.next(K))) {
+			const StringName *L = nullptr;
+			while ((L = p_other->shader_map[*K].next(L))) {
+				set_shader(*L, *K, p_other->shader_map[*K][*L]);
+			}
+		}
+	}
+
+	// Styleboxes.
+	{
+		const StringName *K = nullptr;
+		while ((K = p_other->style_map.next(K))) {
+			const StringName *L = nullptr;
+			while ((L = p_other->style_map[*K].next(L))) {
+				set_stylebox(*L, *K, p_other->style_map[*K][*L]);
+			}
+		}
+	}
+
+	_unfreeze_and_propagate_changes();
+}
+
 void Theme::get_type_list(List<StringName> *p_list) const {
 	ERR_FAIL_NULL(p_list);
 
@@ -1281,6 +1357,7 @@ void Theme::_bind_methods() {
 
 	ClassDB::bind_method("copy_default_theme", &Theme::copy_default_theme);
 	ClassDB::bind_method(D_METHOD("copy_theme", "other"), &Theme::copy_theme);
+	ClassDB::bind_method(D_METHOD("merge_with", "other"), &Theme::merge_with);
 
 	ADD_PROPERTY(PropertyInfo(Variant::OBJECT, "default_font", PROPERTY_HINT_RESOURCE_TYPE, "Font"), "set_default_font", "get_default_font");
 

scene/resources/theme.h

@@ -187,6 +187,7 @@ public:
 
 	void copy_default_theme();
 	void copy_theme(const Ref<Theme> &p_other);
+	void merge_with(const Ref<Theme> &p_other);
 	void clear();
 
 	Theme();